Don't dexpreopt or strip java_test modules

I6bb2c971cee65d2338839753aa0d84939f335b1b accidentally caused
java_test modules to be dexpreopted and possibly stripped.

Test: dexpreopt_test.go
Change-Id: Ida8f046c509c97e38bd3bce66944d32f01530db4

java/java.go

@@ -1601,6 +1601,7 @@ func TestFactory() android.Module {
 		&module.testProperties)
 
 	module.Module.properties.Installable = proptools.BoolPtr(true)
+	module.Module.dexpreopter.isTest = true
 
 	InitJavaModule(module, android.HostAndDeviceSupported)
 	return module